1. Field of the Invention
The invention relates to an extreme UV radiation source device which emits extreme UV radiation, and a process for eliminating debris which forms in an extreme UV radiation source device. The invention relates especially to an extreme UV radiation source device in which the effort is made to prolong its service life by introducing hydrogen radicals, and a process for eliminating contaminations.
2. Description of the Prior Art
According to the miniaturization and increased integration of an integrated semiconductor circuit, an increase of resolution is required in a projection exposure tool when it is being manufactured. To meet this requirement, the wavelengths of the exposure light source are being increasingly shortened. An extreme UV radiation source device (EUV radiation source device) which emits extreme UV radiation (hereinafter also called EUV (extreme ultraviolet) radiation) with wavelengths from 13 nm to 14 nm, especially with a wavelength of 13.5 nm, was developed as a semiconductor exposure light source of the next generation in succession to an excimer laser device.
A few schemes are known for producing EUV radiation in a EUV radiation source device. In one, a high density and high temperature plasma is produced by heating or excitation of a EUV radiating fuel and EUV radiation is extracted from this plasma.
The EUV radiation source device adopting such a scheme based on the method of production of a high density and high temperature plasma is roughly divided into an EUV radiation source device of the LPP (laser produced plasma) type and an EUV radiation source device of the DPP (discharge produced plasma) type (for example, “Current situation and future prospect of research of an EUV (extreme UV) light source for lithography” in J. Plasma Fusion Res. March 2003, Vol. 79, no. 3, pp. 219-260).
In an EUV radiation source device of the LPP type, EUV radiation is used from a high density and high temperature plasma which is formed from irradiated targets such as solids, liquid, gas and the like with a pulsed laser.
On the other hand, in an EUV radiation source device of the DPP type, EUV radiation from a high density and high temperature plasma which has been produced by power current driving is used. Discharge methods in an EUV radiation source of the DPP type, as described in the aforementioned publication, include a Z pinch method, a capillary discharge method, a dense plasma focus method, a hollow cathode triggered Z pinch method and the like.
The EUV radiation source device of the DPP type, as compared to the EUV radiation source device of the LPP type, has the advantages of a small radiation source device and low power consumption of the radiation source system. Practical use in the market is strongly expected.
In the above described EUV radiation source devices of the two types, it is imagined that, currently, roughly decavalent Xe (xenon) ions and Sn (tin) ions are very promising as the radiating fuel which emits EUV radiation with a wavelength of 13.5 nm from a high density and high temperature plasma. Here, tin has a several times higher conversion efficiency than that of xenon. Conversion efficiency is defined as the ratio of the radiation intensity of the EUV radiation with a wavelength of 13.5 nm to the input energy for producing high density and high temperature plasma. Therefore, tin is being noticed as an EUV radiating fuel.
Tin has a melting point of roughly 230° C. The vapor pressure of tin is, however, low. Tin has the property that it does not adequately vaporize before exceeding 2000° C. Therefore, conventionally, tin is supplied to the plasma production area by vaporization of tin by laser irradiation, by self-heating of a tin supply source by a discharge and by similar methods. However, tin, as described above, has a low vapor pressure and is solid at room temperature. Accordingly, when a high density and high temperature plasma is being produced by heating or excitation, there is the disadvantage that a large amount of debris is formed as a result of the tin. Furthermore, tin which has a low vapor pressure deposits in a region with a low temperature within the device when it returns from the plasma state to the normal gaseous state. This degrades the performance of the device.
In an EUV radiation source device, generally, EUV radiation which is emitted by a high density and high temperature plasma is focused once by means of a focusing mirror which is located in the vicinity of the plasma, and afterwards, it is emitted to a subsequent stage. This focusing mirror corresponds, for example, to the above described area with a low temperature within the device. In the case, in which tin and/or a tin compound deposits on the focusing mirror, the reflectivity of the focusing mirror with respect to EUV radiation with a wavelength of 13.5 nm is degraded. As a result, the intensity of the EUV radiation emitted to a subsequent stage decreases. Compare in this respect Japanese Patent Application JP 2004-279246 A (U.S. Patent Application Publication 2004/0183038 A1), Japanese Patent Application JP 2002-504746 A (U.S. Pat. No. 6,359,969 B1), U.S. Patent Application Publication 2003/0190012 A1, International Patent Application Publication WO 2004/092693 A2, Japanese Patent Application JP HEI 10-512092 A (U.S. Pat. No. 5,612,588 A), Japanese Patent Application JP 2003-218025 A (U.S. Pat. No. 6,894,298 B2) and Akira Kanabara, Sputtering Phenomenon, Tokyo University Press, 1984, page 112-117.
As was described above, tin has a several times higher efficiency than that of xenon. It is therefore very promising as an EUV radiating fuel. In tin which has a low vapor pressure and which is solid at normal temperature, in contrast to xenon which is in the gaseous state at a normal temperature and which does not lead to debris, when a high density and high temperature plasma is produced by heating or excitation, a large amount of debris is formed as a result of tin.
Generally, in an EUV radiation source device, between the high density and high temperature plasma and the focusing mirror there is a debris trap which is used to capture debris and to pass only EUV radiation. The debris trap, as described, for example, in Japanese Patent Application JP 2002-504746 A (U.S. Pat. No. 6,359,969 B1), consists of several plates which are arranged in the radial direction of the producing area for plasma with a high temperature and high density. This debris trap captures debris such as metal powder or the like which is formed by sputtering of a metal which is caused in contact with a high density and high temperature plasma, by which the above described plasma is produced, debris as a result of a radiating fuel such as tin or the like, and the like. (The above described metal which is in contact with a high density and high temperature plasma for example in the case of an EUV radiation source device of the LPP type can be defined as a nozzle for supply of the EUV radiating fuel to the plasma producing area and in the case of an EUV radiation source device of the DPP type, a discharge electrode of tungsten or the like).
There is still debris which passes through the debris trap. In particular, most debris as a result of a radiating fuel, such as tin or the like, has a lower weight than the debris of a metallic powder, particles (especially tungsten) or the like (SnH4=122.73, W=183.86) and due to its less linear propagation momentum, it easily passes through the debris trap. This means that the debris of a metallic powder or the like which originates from the vicinity of the light source continues moving after formation with a uniform speed in a certain direction. Therefore, the debris trap which has been formed accordingly is effective for trapping debris of a metallic powder, particles or the like.
On the other hand, debris as a result of a radiating fuel such as tin or the like is in the atomic gaseous state. Since its path is complicated, it often passes through the debris trap. The debris as a result of a radiating fuel, such as tin or the like, which has passed through the debris trap deposits, for example, in the EUV focusing mirror and causes a reduction of the EUV radiation reflection factor of the focusing mirror.
In order to eliminate this disadvantage, in Japanese Patent Application JP 2004-279246 A (U.S. Patent Application Publication 2004/0183038 A1), a process for supplying SnH4 to the plasma producing area was proposed.
The following was named as the advantage of this process:
In contrast to a process in which at room temperature tin as a solid is caused to vaporize, heating to a high temperature is not required.
SnH4 as a gas is easily transported to the heating or excitation space (production space for high density and high temperature plasma) which constitutes the plasma producing area.
Control of the tin concentration is easily carried out by mixing with a rare gas.
In contrast to tin as a solid, SnH4 is normally in the gaseous state. After plasma formation in the producing space for a high density and high temperature plasma, it is therefore easily released in the atomic gaseous state. It hardly agglomerates in the area with a low temperature within the device and hardly deposits.
As a result of the vigorous research and experiments of the inventors, it was found that even by supplying SnH4 as the radiating fuel supply material to the producing area for a high density and high temperature plasma within the EUV radiation source device, deposition of tin and/or a tin compound (for example, a carbide, oxide, or the like) in the area with a low temperature within the EUV radiation source device cannot be completely eliminated.
Tin which has formed from the portion of the SnH4 which does not contribute to plasma formation and/or from the plasma, SnH4 which was formed by a recombination of fragments such as SnH, SnH2, SnH3 (hereinafter SnHx), and fragments of SnHx with a high vapor pressure are released maintaining the gaseous condition by an evacuation means which is spatially connected to the plasma producing area of the EUV radiation source device. However, the following was found.
As a result of plasma formation, metallic clusters such as Sn, Snx (aggregate of atoms and molecules) of the atomic gas produced by decomposition and a portion of the fragments such as SnHx or the like come into contact with the area of low temperature of the device, by which tin and/or tin compounds deposit. For example, SnH4 decomposes on a metallic surface at roughly 150° C., by which a tin film is formed. The term “Sn-compound” for purposes of the present invention is defined, for example, as a carbide, oxide or the like of tin.
It goes without saying that these disadvantages arise in the process for supplying tin as the EUV radiating fuel not only in the case of introducing SnH4 into the plasma producing area, but also in the case of use of another tin hydride with a high vapor pressure, such as Sn2H6 or the like.
In Japanese Patent Application JP 2004-279246 A (U.S. Patent Application Publication 2004/0183038 A1), it is described that hydrogen (H2) gas is introduced in the direction which is linked to the outlet flow which contains debris as a result of a radiating fuel, such as tin or the like, this outlet flow being produced by evacuation operation of the above described evacuation means and being allowed to flow out of the producing space for high temperature and high density plasma. In this way hydrogen (H2) gas can be reacted with vaporous tin, tin hydride with a high vapor pressure is produced, and it is evacuated without deposition within the device.
By using such a process, the effect of debris as a result of a radiating fuel, such as tin or the like, is reduced even more. However, as was described above, the tin layer which has deposited in the area with the low temperature of the device cannot be eliminated by simple introduction of hydrogen (H2) gas. Therefore, if tin and/or a tin compound has deposited on the focusing mirror, the disadvantage of a reduction of the reflectivity of the focusing mirror with respect to the EUV radiation with a wavelength of 13.5 nm and a degradation of the efficiency of the device remain as before.